1. Field of the Invention
The invention relates to a method and the corresponding apparatus for implementing an unpolarized monochromatic light source, and particularly to an arrangement of wavelength tunable capability.
2. The Related Art
In the modern devices testing, such as fiber-optics Dense Wavelength Division Multiplexer (DWDM), Er+3 Doped Fiber Amplifier (EDFA), and Fiber Bragg Granting (FBG), it should be measured with unpolarized light to eliminate the influence of potential polarization dependencies because such testing is suffered due to the high DOP (Degree Of Polarization) of the light source used in the testing. It is appreciated that the DOP is used to characterize the polarization percentage of a light source wherein the DOP of a laser source can be as high as 99% compared with an ideally unpolarized source which substantially approaches to 0%. Unfortunately but factually, the testing instruments, like photodetector module of the inexpensive optical power meter or optical granting of the expensive optical spectrum analyzer, are both substantially intrinsic polarization-dependent, thus resulting in measurement errors thereof. Understandably, using an unpolarized light source can efficiently eliminate the polarization-dependence loss of such polarization-dependent testing instruments.
On the other hand, wave-dependent parameters, e.g., wavelength dependent insertion loss or wavelength dependent gain, are required to precisely characterize the performance of the tested devices or systems in wavelength domain. Therefore, generally there are two type light sources are preferred for this application wherein one is of the wavelength tunable light sources and the other is of the broadband light sources. The former is a tunable laser which may work with the aforementioned inexpensive optical power meter while unfortunately owns high DOP up to 99%, and the latter is an ASE source which owns somewhat the smaller DOP proximate to 5% while the ASE source is expensive and should work with the aforementioned expensive optical spectrum analyzer. In detailed analysis, the ELED (or SLD) and ASE sources are most popular broadband unpolarized light sources which may be chosen to be used with the latter because of the lower DOPs (e.g., 15% for ELED source and 5% for ASE source) to suppress the polarization dependence sensitivity. Anyhow, due to their broadband spectrums, they all have to work with the expensive optical spectrum analyzer in stead of the inexpensive a lower cost optical power meter. Moreover, ELED has the limitation in high dynamic range test for its lower output power, and ASE source is still very expensive today. Regarding the former, such tunable laser being substantially monochromatic for its narrower spectral width so as to work with an inexpensive optical power meter, has wavelength tunability to scan a wide range spectrum and is good for testing, but its high DOP ( greater than 99%) characteristics will suffer the polarization-dependency error.
In brief, the combination of the equipment of either the former (i.e., the tunable laser associated with the optical power meter), or the latter (i.e., the ASE source associated with the optical spectrum analyzer), is still sensitive to input polarization. Therefore, a good unpolarized light source (i.e., the ultra low DOP less than 1% and the low cost), with wavelength tunable capability is desired in today""s instrumentation.
Regarding the method of depolarization, several Pseudo-depolarized methods have been introduced while they are either by using spectrum-averaging method, like Lyot depolarizer (for example, U.S. Pat. Nos. 4,556,293 and 5,692,082), or by time-averaging method, like polarization modulation (for example, U.S. Pat. Nos. 5,359,678 and 4,923,290). For the former, i.e., the spectrum-averaging method, a broadband light sources (e.g., superluminescent diode; SLD) must be chosen wherein the SLD is expensive and has an undesired lower output power. For the latter, i.e., the time-averaging method, a slower detector, compared with the polarization modulating time, should be used while the instantaneous DOP thereof is still unity (DOP=100%) and it is of a so-called active device approach which costs money and more failure possibility due to extra devices involved. Generally, those Pseudo depolarized methods are not true and efficient depolarization methods and still have some incorrectness disadvantages in some circumstances. This is the reason why the parent application (Ser. No. 08/847,177 filed May 1, 1997) presents a so-called polarization-averaging in which the output approaches to an ideal unpolarized light wherein xe2x80x9cLight shows no preferential directional properties when resolved in different directions at right angles to the direction of propagation.xe2x80x9d (see xe2x80x9cPrinciple of Opticsxe2x80x9d, M. Burn and E. Wolf, pp 544-555, 1980).
Therefore, an object of the invention is to provide a newly unpolarized monochromatic light source with wavelength tunable capability so as to open a new way to simplify the complexity to the performance verification of wavelength-dependent testing.
The invention is to provide an unpolarized monochromatic light source with wavelength tunable capability wherein a monochromatic laser source is connected to an incoherent cascaded fiber-ring depolarizer through a single mode fiber under the condition that Ld greater than  greater than LC wherein Ld is the length of fiber-ring delay line and LC is the coherence length of the light source. The combination of the monochromatic laser source and the incoherent cascaded fiber ring depolarizer can be further connected to an external modulator for amplitude modulation, frequency modulation, or phase modulation of the light signal.